memcg, kmem: further deprecate kmem.limit_in_bytes

The deprecation process of kmem.limit_in_bytes started with the commit
0158115 ("memcg, kmem: deprecate kmem.limit_in_bytes") which also
explains in detail the motivation behind the deprecation.  To summarize,
it is the unexpected behavior on hitting the kmem limit.  This patch
moves the deprecation process to the next stage by disallowing to set
the kmem limit.  In future we might just remove the kmem.limit_in_bytes
file completely.

[akpm@linux-foundation.org: s/ENOTSUPP/EOPNOTSUPP/]
[arnd@arndb.de: mark cancel_charge() inline]
  Link: https://lkml.kernel.org/r/20211022070542.679839-1-arnd@kernel.org

Link: https://lkml.kernel.org/r/20211019153408.2916808-1-shakeelb@google.com
Signed-off-by: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: Vasily Averin <vvs@virtuozzo.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/admin-guide/cgroup-v1/memory.rst

@@ -87,10 +87,8 @@ Brief summary of control files.
  memory.oom_control		     set/show oom controls.
  memory.numa_stat		     show the number of memory usage per numa
 				     node
- memory.kmem.limit_in_bytes          set/show hard limit for kernel memory
-                                     This knob is deprecated and shouldn't be
-                                     used. It is planned that this be removed in
-                                     the foreseeable future.
+ memory.kmem.limit_in_bytes          This knob is deprecated and writing to
+                                     it will return -ENOTSUPP.
  memory.kmem.usage_in_bytes          show current kernel memory allocation
  memory.kmem.failcnt                 show the number of kernel memory usage
 				     hits limits
@@ -518,11 +516,6 @@ will be charged as a new owner of it.
   charged file caches. Some out-of-use page caches may keep charged until
   memory pressure happens. If you want to avoid that, force_empty will be useful.
 
-  Also, note that when memory.kmem.limit_in_bytes is set the charges due to
-  kernel pages will still be seen. This is not considered a failure and the
-  write will still return success. In this case, it is expected that
-  memory.kmem.usage_in_bytes == memory.usage_in_bytes.
-
 5.2 stat file
 -------------
 

mm/memcontrol.c

@@ -2771,8 +2771,7 @@ static inline int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	return try_charge_memcg(memcg, gfp_mask, nr_pages);
 }
 
-#if defined(CONFIG_MEMCG_KMEM) || defined(CONFIG_MMU)
-static void cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
+static inline void cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
 {
 	if (mem_cgroup_is_root(memcg))
 		return;
@@ -2781,7 +2780,6 @@ static void cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
 	if (do_memsw_account())
 		page_counter_uncharge(&memcg->memsw, nr_pages);
 }
-#endif
 
 static void commit_charge(struct page *page, struct mem_cgroup *memcg)
 {
@@ -3000,7 +2998,6 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
 static int obj_cgroup_charge_pages(struct obj_cgroup *objcg, gfp_t gfp,
 				   unsigned int nr_pages)
 {
-	struct page_counter *counter;
 	struct mem_cgroup *memcg;
 	int ret;
 
@@ -3010,21 +3007,8 @@ static int obj_cgroup_charge_pages(struct obj_cgroup *objcg, gfp_t gfp,
 	if (ret)
 		goto out;
 
-	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) &&
-	    !page_counter_try_charge(&memcg->kmem, nr_pages, &counter)) {
-
-		/*
-		 * Enforce __GFP_NOFAIL allocation because callers are not
-		 * prepared to see failures and likely do not have any failure
-		 * handling code.
-		 */
-		if (gfp & __GFP_NOFAIL) {
-			page_counter_charge(&memcg->kmem, nr_pages);
-			goto out;
-		}
-		cancel_charge(memcg, nr_pages);
-		ret = -ENOMEM;
-	}
+	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
+		page_counter_charge(&memcg->kmem, nr_pages);
 out:
 	css_put(&memcg->css);
 
@@ -3714,17 +3698,6 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg)
 }
 #endif /* CONFIG_MEMCG_KMEM */
 
-static int memcg_update_kmem_max(struct mem_cgroup *memcg,
-				 unsigned long max)
-{
-	int ret;
-
-	mutex_lock(&memcg_max_mutex);
-	ret = page_counter_set_max(&memcg->kmem, max);
-	mutex_unlock(&memcg_max_mutex);
-	return ret;
-}
-
 static int memcg_update_tcp_max(struct mem_cgroup *memcg, unsigned long max)
 {
 	int ret;
@@ -3790,10 +3763,8 @@ static ssize_t mem_cgroup_write(struct kernfs_open_file *of,
 			ret = mem_cgroup_resize_max(memcg, nr_pages, true);
 			break;
 		case _KMEM:
-			pr_warn_once("kmem.limit_in_bytes is deprecated and will be removed. "
-				     "Please report your usecase to linux-mm@kvack.org if you "
-				     "depend on this functionality.\n");
-			ret = memcg_update_kmem_max(memcg, nr_pages);
+			/* kmem.limit_in_bytes is deprecated. */
+			ret = -EOPNOTSUPP;
 			break;
 		case _TCP:
 			ret = memcg_update_tcp_max(memcg, nr_pages);